This invention relates to imaging devices and is particularly concerned with devices for applying clear, colorless imaging fluids to an opaque open-cell microvoid-containing layer overlying a contrasting substrate.
Several U.S. patents (e.g., Kallock U.S. No. 2,299,991, Larsen U.S. Pat. No. 3,031,328 and Thomas U.S. Pat. No. 3,508,344) disclose composite sheet material wherein a light-colored opaque blushed lacquer layer is coated over a base sheet which is either dark-colored or imprinted with contrasting indicia. The opacity and light color of the blushed lacquer coating are due to the inclusion of numerous microvoids; the local application of (1) heat or pressure (either of which irreversibly collapses the microvoids) or (2) a non-solvent liquid (which fills the microvoids), causes the coating to become selectively transparent or translucent and the underlying backing to become visible. An innocuous non-solvent liquid employed to impart transparency to the opaque microporous layer can subsequently be evaporated to restore the original appearance. A liquid that is a solvent for the lacquer coating would, of course, result in permanent transparency by collapsing the microvoids.
Phillpotts U.S. Pat. No. 2,854,350 describes structures which are functionally similar to those just described, except that the blushed lacquer coatings are replaced by a microporous layer of finely divided calcium carbonate in an organic binder. Transparency is imparted by locally applying pressure or treating selected areas with a wax, oil or grease having a refractive index similar to that of the calcium carbonate. Other pigments may be incorporated in a microporous highly plasticized resin binder; see Hoge et al. U.S. Pat. No. 3,247,006.
It is sometimes desirable to have microvoid-containing sheet material which can be repeatedly transparentized by applying a liquid, but which cannot readily be transparentized by the application of heat or pressure. In such circumstances, a microvoid-containing layer of the type described in Arens U.S. Pat. No. 4,299,880, owned by applicant's assignee, is preferred. This patent discloses a structure in which the microvoid-containing layer consists essentially of particles held in pseudo-sintered juxtaposition by a thermoset binder and has a cohesion value of at least 400 grams force*. FNT *The cohesion value is determined by knife-coating a dispersion of a putative composition on a cleaned gray cold rolled steel panel, drying and curing as appropriate for the composition, to provide a coating 50-60 micrometers thick. Using a "Balance Beam Scrape-Adhesion and Mar Tester", sold by Gardner Laboratories, Inc., Bethesda, Md., a sapphire-tipped stylus is lowered into contact with the test panel and held in fixed position while a ball bearing-supported platform moves the panel. The minimum grams-force required to form a 50-micrometer deep scratch in the coating in a single pass is determined at a magnification of 40.times. and reported as cohesive value.
Products of the type just discussed can be further improved by incorporating in the microvoid layer an organic polymer that jellifies the transparentizing liquid and blocks lateral migration, thus permitting indicia to retain their initial sharpness.
Where the microvoid coating is sufficiently durable (especially, one of the type described in U.S. Pat. No. 4,199,880) it can be reused many times, thus making it attractive for incorporation in student's workbooks, overhead transparencies, computer cards, cards for use as optical character recognition devices, bingo cards, stenographic pads, easel pads, games, etc. Microvoid layers of this type can also be applied to the surface of three-dimensional objects, making it possible to develop such unusual toys as a doll whose apparently pale lips become temporarily rosy-red when a transparentizing "lipstick" is applied to reveal the underlying color. For all applications of this type, it is important that evaporation of the transparentizing liquid be complete, so that the original appearance is restored. As is taught in U.S. Pat. Nos. 4,299,880 and 4,418,098, the specific marking fluid is chosen in large part on the basis of its evaporation rate, which is inversely related to image duration.
There are many ways in which a transparentizing liquid can be applied to the surface of a microvoid-containing layer, e.g., by a stamp pad, typewriter ribbon, sponge, etc., but a particularly preferred imaging device is a pen having a porous nib made of felt, extruded polymer, compressed fiber bundles, etc. Unfortunately, however, when a transparentizing liquid is incorporated in an imaging device, the anticipated number of uses possible is substantially less than would have been predicted. In some instances, as few as two applications of the imaging device to the same area of a given microvoid substrate has resulted in the presence of a "ghost" image that is permanently visible.